1. Field of the Invention
The present invention relates to an image forming apparatus which uses an electrophotographic system such as a copying machine, a printer, or a facsimile and a developing device used therein.
2. Description of the Related Art
As a dry developing method applied to the electrophotographic system, a single-component developing method that uses toner only and a two-component developing method that uses a developer composed of toner and a magnetic carrier are known.
Since the single-component developing method does not use a magnetic carrier, an electrostatic latent image on an image bearing member may not be disturbed by a magnetic brush formed by the magnetic carrier and this method is thus ideal for obtaining high image quality. However, the single-component developing method has a difficulty in stably charging the toner and has a problem in providing stable image quality. Moreover, since this method does not have a medium like a magnetic carrier that conveys the toner, it is difficult to apply uniform conveying force to the toner and the load to the toner is likely to increase. Due to this, deterioration of toner can lead to a decrease in the ability to provide stable image quality. On the other hand, the two-component developing method has a problem in providing high image quality, it is easy to charge the toner and the load to the toner is small, therefore providing highly stable image quality.
In order to solve the problems of both developing methods, a hybrid developing method of applying a conveying bias between a developing roller that bears a two-component developer and a toner bearing member to coat a toner layer on the toner bearing member and developing an electrostatic latent image on an image bearing member with toner to form an image is known.
However, it is known that the hybrid developing method has a difficulty in coating the toner layer stably on the toner bearing member for a long period. This is based on the following reasons.
In the hybrid developing method, toner having a predetermined charge amount (Q/S) is coated on the toner bearing member so as to counteract a potential difference ΔV between the developing roller and the toner bearing member, generated by the conveying bias.ΔV∝Q/S=M/S×Q/M That is, in the hybrid developing method, the amount of coat (M/S) is determined based on the potential difference (ΔV) and the toner charge amount (Q/M). Thus, the amount of coat of toner changes by changing the charging amount of the toner.
In order to solve the problem, the following techniques are known. First, when a toner layer is coated on a toner bearing member, the thickness of the toner layer on the toner bearing member is measured using a toner layer thickness detecting unit. Moreover, a conveying bias between the toner bearing member and the developing roller and a rotation speed of the toner bearing member and the developing roller are changed based on the toner layer thickness, the thickness of the toner layer on the toner bearing member may be controlled to a predetermined thickness (for example, see Japanese Patent Laid-Open No. 2009-008834).
However, since this method uses a toner density sensor or a surface potential sensor as the toner layer thickness detecting unit, the size of the device and associated cost may increase. Moreover, even when the thickness is controlled using the detecting unit, if the conveying bias and the rotation speed of the toner bearing member are changed, it is also necessary to control the conditions of developing between the toner bearing member and the image bearing member on the downstream side at the same time. Due to this, there is a problem that the control is complex and it is difficult to attain an original object and stabilize the amount of toner on the image bearing member.
Thus, a developing method of coating a toner layer stably is proposed (for example, see Japanese Patent Laid-Open No. 60-042776). This method uses a two-component developer composed of a non-magnetic toner and a magnetic carrier where the magnetic carrier is regulated by a magnetic field confined within the developing container. In this manner, it is possible to coat a toner layer on a toner bearing member.
Moreover, a technique of coating a toner layer on a toner bearing member using a rotatable developer regulating member so that the toner is stably charged with a carrier without decreasing the density of an output image and causing the toner to be scattered is disclosed (for example, see Japanese Patent Laid-Open No. 10-198161).
An example of a developing device using this developing method will be described with reference to FIGS. 26A and 26B. FIGS. 26A and 26B are diagrams illustrating a schematic configuration of a developing device of the related art.
A developing device 120 of this example includes a developing container 121 that stores a developer composed of toner and a magnetic carrier. A developing roller 142 that is rotatable in the direction indicated by an arrow in the drawings and a regulating sleeve 143 as a developer regulating member disposed at a predetermined distance above the developing roller 142 are disposed in an opening of the developing container 121 formed at such a position that it faces the image bearing member 101.
The regulating sleeve 143 is formed of a non-magnetic member and is arranged so as to be rotatable in the same direction as the rotation direction of the developing roller 142, and a permanent magnet 144 is fixedly disposed inside the regulating sleeve 143.
Further, a conveying member 126 that rotates in the direction indicated by the arrows in the drawings mix the developer in the developing container 121 and supply the developer to the developing roller 142 in the developing container 121.
Coating of a toner layer on the developing roller 142 in the developing device 120 will be described. The developer in the developing container 121 is mixed by the conveying member 126 and is supplied to the developing roller 142. The developer supplied is born on and conveyed by the developing roller 142, magnetized by the magnetic force of the permanent magnet 144 in the regulating sleeve 143 and regulated in a developer regulating region (G). Details of the developer regulating region (G) are illustrated in FIG. 26B. FIG. 26B is an enlarged view of the developer regulating region (G) in FIG. 26A.
A magnetic carrier in the developer regulated by the magnetic field in the developer regulating region (G) is restricted by the magnetic force of the permanent magnet 144 in the regulating sleeve 143. Since the regulating sleeve 143 rotates in the direction indicated by an arrow illustrated in the drawings, the magnetic carrier is subject to a conveying force in the direction (A) returning it to the developing container 121. Thus, the magnetic carrier is sequentially returned to the developing container 121 by the conveying force from the regulating sleeve 143 while being restricted in the developer regulating region (G). Due to this, the magnetic carrier circulates in the developing container as indicated by an arrow in the drawings without leaking to a developing portion that faces the image bearing member 101.
On the other hand, the non-magnetic toner in the developer regulating region (G) is not restricted by the magnetic field in the developer regulating region (G). Moreover, a mirroring force from the developing roller 142, being caused by the charge formed by the frictional charging between the magnetic carrier and the surface of the developing roller 142, acts on the non-magnetic toner. Thus, the non-magnetic toner is subject to the conveying force in the rotation direction (B) of the developing roller 142 with the rotation of the developing roller 142 and passes through the developer regulating region (G) so that the non-magnetic toner is coated on the developing roller 142.
In this manner, only the non-magnetic toner that is sufficiently charged is coated on the developing roller 142 while preventing the leakage of the magnetic carrier.
Since the developing methods disclosed in Japanese Patent Laid-Open No. 60-042776 and Japanese Patent Laid-Open No. 10-198161 take a configuration in which the toner bearing member is coated by the toner which makes physical contact with the toner bearing member, the amount of coat will not vary greatly with a variation in the toner charge amount (Q/M) like the hybrid developing method. Specifically, in the hybrid developing method, the amount of coat increases when the toner charge amount decreases. In contrast, the developing methods disclosed in Japanese Patent Laid-Open No. 60-042776 and Japanese Patent Laid-Open No. 10-198161 can suppress an increase in the amount of coat. That is, since these developing methods can suppress the variation in the amount of coat with the variation in the toner charging amount, it is possible to suppress the variation in the image density.
Although these methods can suppress the variation in the toner layer thickness in a vertical direction, it is found that the arrangement position of the toner coat in a plane is not uniform but is uneven. In this case, a region in which toner is not coated is present approximately in parallel to the rotation direction of the developing roller, and an unevenness occurs in the toner layer on the surface of the developing roller.
It is therefore difficult to form a toner layer uniformly on the developing roller which may directly lead to image defects. This problem is particularly evident when the toner layer is thin.
When the toner layer includes a plurality of layers, even if the toner layer on the toner bearing member is slightly non-uniform, a toner image on the image bearing member is made uniform by an AC bias applied and is rarely identified as an image defect.
On the other hand, when the toner layer includes substantially a single layer, it is not possible to prevent image defects due to the above effect. This is because, although the toner image can be made uniform, since an absolute toner amount is not sufficient, a portion where the sheet is not completely coated with toner may occur, and an allowable density unevenness is not attained. As a result, the portion is identified as an image defect.
The problem results from the following mechanism. This mechanism will be described with reference to FIGS. 27A and 27B. FIGS. 27A and 27B are diagrams for describing the mechanism of the problem of the related art. FIG. 27A schematically illustrates the toner layer coated on the developing roller and the magnetic brush returned to the developing container by the regulating sleeve in the developer regulating region (G).
Although a large amount of magnetic brushes are conveyed by the developing roller, and a large amount of magnetic brushes present in the developer regulating region (G) are returned to the developing container by the regulating sleeve, some magnetic brushes are not illustrated.
As in FIG. 27A, the toner layer coated on the developing roller is disturbed by the magnetic brush conveyed by the regulating sleeve when the toner layer passes through the developer regulating region (G). Moreover, in order for the toner to uniformly make contact with the surface of the developing roller, it is necessary to apply a sufficient relative difference to a moving speed of the surface of the developing roller and the conveying speed of the developer on the developing roller. This is to secure a sufficiently high contact frequency for allowing the surface of the developing roller to make contact with the toner coated on the magnetic carrier.
Due to this, the coated toner layer is also disturbed by a subsequent magnetic brush that is conveyed by the developing roller and passes the toner layer as well as the magnetic brush that is conveyed by the regulating sleeve. FIG. 27B is a diagram schematically illustrating a state where the toner coated on the developing roller is disturbed by the magnetic brush.
As illustrated in FIG. 27B, when the magnetic brush collides with the toner coated on the developing roller, the toner moves or rotates on the developing roller, and the adhering force (mirroring force) of the toner in relation to the developing roller decreases.
In this case, since a magnetic carrier at the tip of the magnetic brush has toner coated on the developing roller on the downstream side, the magnetic carrier is charged with a reverse polarity by a charge amount of toner consumed. Due to this, the toner coated on the developing roller is scraped by the magnetic carrier when the toner passes through the developer regulating region (G).
Due to the reasons, a scrape mark of the magnetic carrier appears in parallel to the moving direction of the magnetic brush (that is, mainly the rotation direction of the developing roller and the regulating sleeve), and it is not possible to coat a uniform toner layer.